1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to a system and method for using an internal filter of a delta-sigma modulator to provide integration and decimation functions.
2. Description of the Related Art
Many signal processing systems, such as audio signal processing systems, utilize delta-sigma modulators to provide output data with a high, in-band signal to noise ratio (“SNR”). FIG. 1 depicts a conventional digital signal processing (“DSP”) system 100 for processing a signal source 102 into a digital signal. For example, the DSP system 100 can be used to perform high quality encoding of super audio compact disk (“SACD”) data. Analog-to-digital converter (“ADC”) 104 receives an input signal from the signal source 102 and converts the signal into a digital signal represented by discrete digital data sampled at a particular sampling frequency, fs. It is well known that operating at large multiples of the sampling frequency spreads typical noise signals across a larger frequency band, thus improving SNR. This is particularly true for audio signals because audio signals reside in a relatively small baseband between 0 Hz and 25 kHz. Currently, a typical ADC 104 operates at 128 times (“*”) fs, and operational frequencies are expected to increase to 256*fs and beyond over time.
Referring to FIGS. 1 and 2, some DSP systems utilize one-bit output data and others utilize multi-bit output data. For example, SACD uses one-bit output data to encode high fidelity audio signals. The ADC 104 typically provides a multi-bit output. DSP system 100 utilizes delta-sigma modulator 108 to provide a high, in-band SNR and to provide a one-bit output when desired. The delta-sigma modulator 108 operates at a lower frequency than ADC 104, such as 64*fs. Additionally, as depicted in FIG. 2, much of the noise energy 202 in the output data of ADC 104 resides near the Nyquist frequency. Thus, decimation filter 106 down samples the output data from ADC 104 and provides a low-pass filter with a frequency response. Without low-pass filtering of the output data from ADC 104, significant noise energy near the pre-decimation Nyquist frequency would fold down into the signal baseband.
Delta-sigma modulator 108 includes an L-order loop filter 110 that includes a series of integration stages to shape the noise in the decimated output data out of the baseband. Delta-sigma modulator 108 also includes a 1 or multi-bit quantizer 112 to provide a series of output bits representative of the baseband input signal by quantizing the output data of loop filter 110 and providing feedback to loop filter 110 in a well-known manner. “Delta-sigma modulators” are also commonly referred to using other interchangeable terms such as “sigma-delta modulators”, “delta-sigma converters”, “sigma delta converters”, and “noise shapers”.
FIG. 3 depicts a simulated response of DSP system 100 in the frequency domain when decimating from 128*fs to 64*fs for 128*fs equal to approximately 6 MHz. The z-domain transfer function, H(z), of decimation filter 106 is H(z)=(1+z−1)/2, which represents a two sample averaging filter. Other transfer functions can be used. The peak quantization noise 302 near the Nyquist frequency of 3 MHz dropped to approximately −40 dB. FIG. 4 depicts an amplified view of the peak quantization noise 302 in the top 20 kHz, which is the energy that would be folded back due to aliasing.
Several problems exist with DSP system 100. For example, use of the decimation filter 106 requires additional processing resources that could be used for other operations and/or occupy additional integrated chip area. Additional noise reduction of noise that will fold into the baseband is advantageous.